Embryonic antigens (EA) are shared by many species, including mice and humans. Tumors of all metazoans tested carry EA, and EA expression seems to be a ubiquitous trait of transformed cells. EA serves as an autoantigen in syngeneic animal systems when present on developing embryos or on tumors, and these antigens elicit a broad array of autochthonous immunological responses in the host which are modestly understood. Methods for preparing monoclonal antibodies to hamster EA determinants shared with those of mouse and human fetuses have been developed, and several monoclones with suitable EA specificity have been prepared. The monoclonal antibodies are being used to: (1) purify and characterize EA species in rodent fetuses and on rodent tumors; and (2) discover the spectrum of human fetal and tumor tissues which exhibit these antigens in pregnant or tumor-bearing hosts. A catalog of these EA species and the distribution of EA in rodent and human fetuses will be prepared.